of the relation of capacity, self-induction and frequency as regards
the general result. The effects of capacity are the most striking, for
in these experiments, since the self-induction and frequency both are
high, the critical capacity is very small, and need be but slightly
varied to produce a very considerable change. The experimenter may
bring his body in contact with the terminals of the secondary of the
coil, or attach to one or both terminals insulated bodies of very
small bulk, such as bulbs, and he may produce a considerable rise or
fall of potential, and greatly affect the flow of the current through
the primary. In the experiment before shown, in which a brush appears
at a wire attached to one terminal, and the wire is vibrated when the
experimenter brings his insulated body in contact with the other
terminal of the coil, the sudden rise of potential was made evident.
I may show you the behavior of the coil in another manner which
possesses a feature of some interest. I have here a little light fan
of aluminium sheet, fastened to a needle and arranged to rotate freely
in a metal piece screwed to one of the terminals of the coil. When the
coil is set to work, the molecules of the air are rhythmically
attracted and repelled. As the force with which they are repelled is
greater than that with which they are attracted, it results that there
is a repulsion exerted on the surfaces of the fan. If the fan were
made simply of a metal sheet, the repulsion would be equal on the
opposite sides, and would produce no effect. But if one of the
opposing surfaces is screened, or if, generally speaking, the
bombardment on this side is weakened in some way or other, there
remains the repulsion exerted upon the other, and the fan is set in
rotation. The screening is best effected by fastening upon one of the
opposing sides of the fan insulated conducting coatings, or, if the
fan is made in the shape of an ordinary propeller screw, by fastening
on one side, and close to it, an insulated metal plate. The static
screen may, however, be omitted, and simply a thickness of insulating
material fastened to one of the sides of the fan.
To show the behavior of the coil, the fan may be placed upon the
terminal and it will readily rotate when the coil is operated by
currents of very high frequency. With a steady potential, of course,
and even with alternating currents of very low frequency, it would not
turn, because of the very slow exchange of air and, consequently,
smaller bombardment; but in the latter case it might turn if the
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